A lithographic printing plate has been prepared by a conventional method comprising a process of exposing a transparent original (e.g., a negative or positive film) in contact with a printing plate such as an aluminum plate or zinc plate each coated thereon with a photosensitive layer and developing the exposed plate. A positive-working photosensitive lithographic printing plate, for example, comprises a hydrophilic support having thereon a photosensitive ink-affinity layer capable of being solubilized upon imagewise-exposure to an actinic ray, such as ultraviolet radiation. When exposed and developed with an alkaline developer, an imaging area (nonexposed portion) remains and a non-imaging area (exposed portion) is removed to expose the hydrophilic surface of the support. As a result, the non-imaging area is capable of retaining water and thereby it repulses ink. On the other hand, the area (imaging area) which has not been removed through development, is hydrophobic so that it repulses water and becomes ink-receptible. The difference between the hydrophilic nonimaging area and the hydrophobic imaging area is employed in common lithographic printing. In a negative-working photosensitive lithographic printing plate, on the contrary, a photosensitive layer in an exposed portion (imaging area) is hardened and a photosensitive layer in an unexposed portion (non-imaging area) is removed by a developer so that it can be used as a lithographic printing plate.
With regard to editing the composition of a printing material which is a process prior to the preparing process of the printing plate, computer typesetting systems (CTS) made up of a combination of an automatic typesetting machine and computer, electronic composing and DTP (Desk Top Publishing) have become popular with the spread of word processors and personal computers. The computer generated data thus-edited is usually prepared as an original forme of a photographic film. If a printing plate material is available which enables making the printing plate directly from the output data of a CTS or aother electronic edting device, it is possible to shorten the printing process and also save intermediate materials. As a technique in response to the direct plate-making, it has been reported that there has been developed a plate-making material using an organic semiconductor or silver salt photographic light sensitive material. However, as it still has defects such as needing a new processing step and the plate-life is insufficient, it has not yet become an acceptable substitute for pre-sensitized plates.
Thermal recording, in which an image canbe obtained directly through thermal energy of an input signal without developing and fixing, is a dry-processing system without intermediate chemical steps, therefore, it is broadly employed in facsimile and printers. Recently, there has been employed a thermal transfer recording system, in which thermal transfer recording can be accomplished by exposing a thermal transfer recording material to a laser beam which converts the laser beam to heat in the recording material. In this laser thermal-transfer recording system, the laser beam, used as energy supply can be condensed to such an extent that highly-resolution recording becomes feasible, leading to use in the printing of art works.
With respect to the image forming technique using lasers, there have been proposed applications for direct thermographic plate-making. In Japanese Patent examined No. 51-6568 is disclosed a technique, in which vapor-deposition coating of metal such as aluminum or copper, or a coating comprised of carbon black dispersed in an organic solvent-soluble binder is usable as an ablatable light-shielding layer.
However, in the case when a metal depositin coating is used as the ablatable light-shielding coating, it is problematic in that two wet-processing steps are required comprising removing a masking layer in an exposed portion and developing the photosensitive layer. Further, since a large apparatus for vapor-deposition is required for forming a vapor-deposited coating over a photosensitive layer on an industrial scale, it is difficult to provide it at a competitive low price. In cases when a coating of an organic solvent-soluble binder such as nitrocellulose is used as the ablatable light-shielding layer, on the other hand, the ablatable light-shielding layer can be coated using a technique similar to that of the photosensitive layer. However, while coating the shielding layer, the surface of the photosensitive layer is dissolved by an organic solvent contained in a coating solution and mixed with a part of the shielding layer, producing such problems that sensitivity of the photosensitive layer is likely to be decreased or printing stains are caused due to the incomplete removal of the shielding layer during development.
In view of the foregoing, the present invention was developed. Therefore, an object of the present invention is to provide a method of preparing a lithographic printing plate directly from a digital image signal without the use of any intermediate material in the plate-making process. Another object of the invention is to provide a technique for overcoming the above-described problems, so that, in a method for preparing a lithographic printing plate directly from a digital image signal, it is to provide a technique whereby an ablatable light-shielding layer can be readily formed at a low cost on an industrial scale, a process of removing only masking layer in an unexposed portion is not required, there is no adverse effect of the light-shielding layer on the photosensitive layer and any unablated portion of the light-shielding layer can be sufficiently removed during the developing process.